Hydrocarbon cracking process

ABSTRACT

A process for cracking a feed comprising at least one alkane involving contacting said feed with H 2  S and a high surface area contact material under cracking conditions.

BACKGROUND OF THE INVENTION

The present invention relates to the cracking of light hydrocarbons. Inanother aspect, the present invention relates to a method of increasingthe conversion and in some cases the selectivity obtained during thecracking of light hydrocarbons.

It is well known that the product distributions obtained in the crackingof hydrocarbons are non-selective and, even at low conversions, producea large number of primary products. Obviously, it would be preferable toobtain more selectivity to the specific desired products since suchwould give greater yields of the desired product and would in many casesmake separation of the desired product less expensive.

In addition to poor selectivity, thermal cracking reactions are alsoknown to require large inputs of energy to achieve high conversionlevels. Accordingly, there is a need to increase the conversion level ofsuch processes so that one can either use less energy or make more ofthe desired product in order to counterbalance the energy costs.

It has been known for several years that H₂ S can change the conversionlevel of hydrocarbon pyrolysis reactions and alter the selectivity tovarious products. Theories for explaining the effect of the H₂ S arepresented in Scacchi et al, Int. J. Chem. Kinetics, 2, 115 (1970); Saigeet al, C. R. Acad. Sc. Paris, 274, 322 (1972); Rebick, Frontiers of FreeRadical Chemistry, Academic Press, Inc. (1980); and Rebick, Ind. Eng.Chem. Fundam, 20, 54 (1981). The present invention is based upon thediscovery that the cracking of light hydrocarbons in the presence of H₂S and certain high surface area materials increases the conversion farbeyond what one would expect from the effects of the H₂ S or the highsurface area material alone.

SUMMARY OF THE INVENTION

In accordance with the present invention, a hydrocarbon feed comprisingat least one alkane having 2 to 20 carbon atoms per molecule iscontacted under cracking conditions with H₂ S and a solid contactmaterial comprising silica having a surface are of at least 50 m² /gram.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graphical comparison of the relative effects of H₂ S oncracking carried out in the presence of low and high surface areamaterials.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is expected to provide at least some improvementin the cracking of any alkanes. However, since the cracking of highermolecular weight materials generally requires the employment oftemperatures below those which give substantial decomposition of the H₂S, the invention is most useful in the cracking of alkanes having nomore than 20 carbon atoms per molecule. The invention is especiallyuseful in cracking alkanes containing 2 to 12 carbon atoms per molecule.Preferably, the feed consists essentially of hydrocarbons. Since thepresent invention has not been found to increase the cracking ofolefins, the preferred feeds are those in which alkanes are the majorhydrocarbon. More preferably, the feed consists essentially ofhydrocarbons and contains at least 80 volume percent alkanes. Theincreased cracking is more notable for those alkanes having at least 4carbons per molecule.

Any suitable cracking conditions can be employed and they will of coursevary somewhat depending upon the nature of the hydrocarbon-containingfeed. Typically though, the cracking will be conducted at a temperaturein the range of about 400° C. to about 900° C., more preferably about500° to about 800° C.

The currently preferred high surface area contact materials are silicagel. The contact material can have associated therewith othercatalytically active material. Obviously, however, if the H₂ S adverselyaffects the activity of the catalytically active contact material thenone does not obtain the advantages of this invention. The form in whichthe contact material is employed does not appear to significantly affectthe observed benefits. In lab scale work, it has been common to use20-40 mesh particles. In commercial scale work even 1/8 inch pelletshave proven useful.

The amount of H₂ S employed can vary over a wide range. Typically the H₂S will be employed in an amount in the range of 0.1 to 10 mole percent,more preferably 1 to 3 mole percent, based on the moles of alkane in thehydrocarbon feed. Most preferably the H₂ S is employed in an amountgreater than that needed for substantially inhibiting carbon formationresulting from the presence of materials that tend to encourage carbonformation. The determination of the amount of H₂ S needed tosubstantially inhibit carbon formation can be readily determined for anyselected cracking conditions by evaluating several H₂ S levels andnoting the level at which there is no additional significant decrease incarbon formation. Typically after H₂ S has been passed through thereaction zone for some period of time there will be no additionalsignificant decrease in the level of carbon formation. Thus, no matterwhat level of H₂ S is selected after enough has passed through thereaction zone one is carrying out the reaction in the absence ofmaterials that are in a form that would cause any significant amount ofcarbon formation if the H₂ S were not employed. Once that point isreached then any level of H₂ S is obviously greater than that needed tosubstantially inactivate carbon formation.

It is theorized that the surprising improvement in cracking obtainedover high surface area contact material is due to the fact that thehigher surface area material acts as a catalyst for the decomposition ofthe H₂ S. Accordingly, the contact time for the reaction can affect theresults observed. Typically, the hydrocarbon feed is passed in contactwith the contact material at a rate of about 100 to 4000 volumes ofgaseous hydrocarbon feed per volume of contact material per hour, ormore preferably 500 to 2500.

In some cases, particularly in small scale reactions, it is desirable toemploy an inert diluent in conjunction with the hydrocarbon feed and theH₂ S. The typical preferred diluent is nitrogen. Generally when thediluent is employed, it is employed in an amount no greater than about 3times the combined volumes of the hydrocarbon feed and the H₂ S.

The present invention and its benefits will be further illustrated bythe following examples.

EXAMPLE I

This example illustrates the experimental setup for investigating thethermal cracking (pyrolysis) of alkanes. The reactor was a quartz tubehaving an outer diameter of about 8 mm and a length of 25 cm. It wasfilled with a single fixed bed of refractory oxide contact materialabout 6-10 cm high. The reactor was heated with a thermostaticallycontrolled external heater. The reactor temperature was measured in thecenter of the catalyst bed by means of a thermocouple enclosed in anaxial thermocouple well extending into the refractory oxide bed. Threefeed streams were introduced into the reactor: various alkanes (eitherPhillips Petroleum Company pure grade or Matheson Gas Products researchgrade), a mixture of 10-20 mole percent of H₂ S (Matheson CP grade) and80-90 mole percent of N₂, and air during the regeneration of the beds.These feed streams were introduced through the separate stainless steelfeedlines each equipped with a flow meter, a flow control valve and anoverpressure shutoff valve. The feedlines joined in a mixing T equippedwith a pressure gauge and an overpressure control interfaced with theabove-mentioned shutoff valves. The mixed feed streams, under a pressureof about 1 atm entered the reactor from the top.

The reactor effluent stream passed through an ice cooled trap, whereliquid components were condensed. The gaseous components were usuallysnap sampled every two minutes and were analyzed for hydrocarbons (notfor hydrogen) with a Perkin Elmer Sigma 3 chromatograph. Liquid sampleswere analyzed at the end of each run with a Hewlett Packard 5880chromatograph containing a 50 ft OV-101 glass capillary column.

Data from the chromatograph were evaluated and expressed in terms of%-conversion (moles of converted feed hydrocarbon in effluent÷moles offeed introduced×100), %-yield (moles of a specific product÷moles of feedintroduced×100), and %-selectivity (yield÷conversion×100).

EXAMPLE II

Results of 14 representative pyrolysis runs employing n-butane plus,when desired, a mixture of H₂ S and N₂ and various refractory oxides ofvarying surface area are summarized in Table I. In runs employing H₂ Sits concentration was 1 mole-% of the alkane feed.

                                      TABLE I                                     __________________________________________________________________________                         With H.sub.2 S         Relative                          Without H.sub.2 S                    Difference in                                                                        Increase in                                  Temp.                                                                             Conversion  Temp.                                                                             Conversion                                                                          Conversion                                                                           Conversion                        Refractory                                                                          Run  (°C.)                                                                      (%)   Run   (°C.)                                                                      (%)   (%)    (%)                               __________________________________________________________________________    None  1    641  3.2  2     635  4.9  1.7    53                                      (Control)                                                                          666  6.3  (Control)                                                                           658  8.8  2.5    40                                           688 11.0        680 14.6  3.6    33                                           713 25.5        704 21.6  --     --                                           742 36.4        747 44.7  8.3    23                                           763 48.2        766 57.3  9.1    19                                           781 59.9        785 69.6  9.7    16                                           800 72.0        806 81.7  9.7    13                                           819 83.3        824 91.5  8.2    10                                Quartz                                                                              3    682  6.8  4     682  9.6  2.8    41                                Chips (Control)                                                                          706 12.8  (Control)                                                                           702 17.0  4.2    33                                           728 21.2        723 26.7  5.5    26                                           750 32.9        745 40.3  7.4    22                                           771 47.2        768 56.6  9.4    20                                           792 62.2        789 71.6  9.4    15                                           812 76.4        809 83.7  7.3    10                                Silica                                                                              5    657 10.1  6     658 16.8  6.8    67                                (Surface                                                                            (Control)                                                                          684 13.0  (Invention)                                                                         682 28.6  15.6   120                               Area =     706 21.7        706 46.2  24.5   113                               317 m.sup.2 /g)                                                                          727 29.9        724 64.5  34.6   116                                          748 44.5        742 74.0  29.5   66                                           768 62.3        768 90.1  27.8   45                                Silica                                                                              7    665  8.5  8     662 22.9  14.4   170                               (Surface                                                                            (Control)                                                                          730 31.7  (Invention)                                                                         723 61.1  29.4   93                                Area =     753 49.6        748 79.8  30.2   61                                314 m.sup.2 /g)                                                                          773 63.0        769 89.4  26.4   42                                           791 77.1        786 94.0  16.9   22                                Silica                                                                              9    663  2.9  10    661  7.3  4.4    152                               (Surface                                                                            (Control)                                                                          687  5.4  (Invention)                                                                         684 20.1  14.7   272                               Area =     710 11.3        711 41.6  30.3   268                               185 m.sup.2 /g)                                                                          752 32.9        747 76.6  43.7   133                                          773 49.7        769 90.0  40.3   81                                           792 66.8        791 96.7  29.9   45                                Silica                                                                              11   665  9.1  12    655 18.1  9.0    99                                (Surface                                                                            (Control)                                                                          690 14.5  (Invention)                                                                         686 25.5  11.0   76                                Area =     717 25.1        708 38.0  12.9   51                                84.7 m.sup.2 /g)                                                                         731 36.1        729 50.4  14.3   40                                           751 42.8        751 66.9  24.1   56                                           774 67.5        766 82.8  15.3   23                                Silica                                                                              13   661 10.1  14    661 18.4  8.3    82                                (Surface                                                                            (Control)                                                                          689 13.1  (Invention)                                                                         684 27.3  14.2   108                               Area =     710 20.5        707 54.2  33.7   164                               56.8 m.sup.2 /g)                                                                         730 30.7        734 64.5  33.8   110                                          751 42.9        749 77.1  34.2   80                                           772 59.7        768 74.7  15.0   25                                           792 73.8        785 89.4  15.6   21                                __________________________________________________________________________

Data in Table I show that at comparable reactor temperatures (660°-800°C.) and flow rates (200 cc/min n-C₄ and 200 cc/min N₂) the presence of 1mole-% of H₂ S in the feed always caused an increase in n-butaneconversion. However, this increase in conversion, both in absolute andrelative terms, was unexpectedly much larger (20-270%; see Runs 5-14) inruns employing amorphorus SiO₂ (surface area: 57-317 m² /g, determinedby BET N₂ adsorption) than in runs employing low surface area quartzchips (16-40 mesh) or no catalyst packing at all (4-44% increase inconversion; seen Runs 1-4). This unexpected difference in the effect ofH₂ S on n-butane conversion is graphically illustrated for four of the14 runs in FIG. 1. It is believed that there is an interaction betweenH₂ S and high surface area amorphous SiO₂, which is absent in lowsurface area crystalline SiO₂ such as quartz, and that this interactionunexpectedly promotes the pyrolysis of n-butane.

EXAMPLE III

This example illustrates another unexpected effect of H₂ S plusamorphous, high surface silica on the pyrolysis of n-butane. Results ofdetailed analysis of reactor effluents produced on silica with anwithout H₂ S, each at a temperature selected to yield 80% conversion,are summarized in Table II.

                  TABLE II                                                        ______________________________________                                                   Run 15 (Control)                                                                             Run 16 (Invention)                                             Silica         Silica                                              Refractory (SA: 185 m.sup.2 /g)                                                                         (SA: 185 m.sup.2 /g)                                Amount of H.sub.2 S                                                                      0              1 volume or mole %                                  Weight-%   80%            80%                                                 Temperature                                                                              800° C. 753° C.                                      (°C.)                                                                             Weight-%  Mole-%   Weight-%                                                                              Mole-%                                  ______________________________________                                        n-Butane   20.8      10.8     20.4    11.6                                    Isobutane  0.3       0.2      0.4     0.2                                     Butenes    6.9       2.2      6.0     3.6                                     Butadiene  1.5       0.9      1.3     0.8                                     Propane    1.8       0.3      1.3     1.0                                     Propylene  23.7      18.3     35.0    28.6                                    Ethane     4.2       4.4      8.1     9.0                                     Ethylene   29.0      32.3     12.1    14.5                                    Methane    14.7      30.5     14.7    30.6                                    ______________________________________                                    

Data in Table II show two effects:

(a) the dehydrogenation of n-butane to butenes and butadiene is only aminor side reaction, and about 90% by weight of the products containless than 4 C-atoms and are therefore formed by thermal cracking;

(b) unexpectedly the amount of propylene was considerably higher and theamount of ethylene was considerably lower when n-butane was pyrolyzed inthe presence of amorphous silica plus H₂ S rather than on silica alone.

EXAMPLE IV

This example illustrates that the unexpected effect of silica plus H₂ Son the conversion of n-butane described in Example II was also observedfor other alkanes. Table III summarizes conversion data for ethane,propane, isobutane and n-decane on low surface, crystalline SiO₂ (quartzchips) with an without H₂ S and on high surface, amorphous silica withan without H₂ S, each at the same temperature and feed flow rateconditions.

                                      TABLE III                                   __________________________________________________________________________                             H.sub.2 S Relative                                                 Temp       Added                                                                             Conversion                                                                          Change in                                  Run     Feedstock                                                                           °C.                                                                        Refractory                                                                           (Vol-%)                                                                           (%)   Conversion (%)                             __________________________________________________________________________    17 (Control)                                                                          Ethane                                                                              800 Quartz Chips                                                                         0   36    -11                                        18 (Control)                                                                          Ethane                                                                              800 Quartz Chips                                                                         1   32                                               19 (Control)                                                                          Ethane                                                                              800 Silica 0   48    +46                                        20 (Invention)                                                                        Ethane                                                                              800 Silica 1   70                                               21 (Control)                                                                          Propane                                                                             775 Quartz Chips                                                                         0   40    +13                                        22 (Control)                                                                          Propane                                                                             775 Quartz Chips                                                                         1   45                                               23 (Control)                                                                          Propane                                                                             775 Silica 0   41    +24                                        24 (Invention)                                                                        Propane                                                                             775 Silica 1   57                                               25 (Control)                                                                          Isobutane                                                                           750 Quartz Chips                                                                         0   38    +18                                        26 (Control)                                                                          Isobutane                                                                           750 Quartz Chips                                                                         1   45                                               27 (Control)                                                                          Isobutane                                                                           750 Silica 0   37    +65                                        28 (Invention)                                                                        Isobutane                                                                           750 Silica 1   61                                               29 (Control)                                                                          n-Decane                                                                            670 Quartz Chips                                                                         0   16    +50                                        30 (Control)                                                                          n-Decane                                                                            670 Quartz Chips                                                                         1   24                                               31 (Control)                                                                          n-Decane                                                                            670 Silica 0   22     +145                                      32 (Invention)                                                                        n-Decane                                                                            670 Silica 1   54                                               __________________________________________________________________________

Unexpectedly, the change in alkane conversion caused by 1 volume % of H₂S was consistently higher with high surface silica (surface area: 317 m²/g) than with quartz chips. Detailed analytical data for Runs 29, 30, 31and 32 are summarized in Table IV.

                                      TABLE IV                                    __________________________________________________________________________                    Quartz Chips                                                                             Silica                                                             29.sup.(1)                                                                         30.sup.(2)                                                                          31.sup.(1)                                                                         32.sup.(2)                                    Product Component                                                                             (Control)                                                                          (Control)                                                                           (Control)                                                                          (Invention)                                   __________________________________________________________________________    C.sub.10+                                                                           (g per 100 g feed)                                                                      0.15 0.17  0.17 --                                            C.sub.10                                                                            (g per 100 g feed)                                                                      84.3 76.47 77.90                                                                              46.20                                         C.sub.8+9                                                                           (g per 100 g feed)                                                                      0.68 2.75  1.88 4.56                                          C.sub.7                                                                             (g per 100 g feed)                                                                      1.56 2.07  1.51 5.07                                          C.sub.6                                                                             (g per 100 g feed)                                                                      1.59 1.89  1.43 3.75                                          C.sub.5                                                                             (g per 100 g feed)                                                                      1.41 0.95  0.84 2.24                                          C.sub.4                                                                             (g per 100 g feed)                                                                      2.50 2.82  2.47 7.95                                          Propylene                                                                           (g per 100 g feed)                                                                      1.82 3.10  3.27 7.29                                          Propane                                                                             (g per 100 g feed)                                                                      0.10 0.23  0.13 2.31                                          Ethylene                                                                            (g per 100 g feed)                                                                      3.90 6.14  6.38 10.10                                         Ethane                                                                              (g per 100 g feed)                                                                      1.01 1.92  1.31 5.73                                          Methane                                                                             (g per 100 g feed)                                                                      0.97 1.60  1.53 3.06                                          __________________________________________________________________________     .sup.(1) flow rate was 1.18 g/minute ndecane, 203 cc/minute nitrogen          .sup.(2) flow rate was 1.18 g/minute ndecane, 170 cc/minute nitrogen and      30 cc/minute 13% H.sub.2 S in nitrogen.                                  

Data in Table IV show that at 670±5° C. the H₂ S over the silicaproduced a greater increase in C₄ to C₇ hydrocarbon production than theH₂ S over the quartz chips.

EXAMPLE V

This example illustrates the pyrolysis of n-butane on silica (surfacearea: 185 m² /g) containing 10% by weight of transition metals, with andwithout H₂ S. In the runs using no H₂ S, the transition metals wereemployed as oxides. In the runs using H₂ S, the catalysts werepretreated so that they were in the sulfide form prior to use in thecracking. Conversions and selectivities are summarized in Table V.

                  TABLE V                                                         ______________________________________                                                                            Propylene                                                  H.sub.2 S Added                                                                          Conversion                                                                            Selectivity                               Run  Catalyst    (Mole-%)   (%)     (%)                                       ______________________________________                                        33   Mo on Silica                                                                              0          48      37                                        34   Mo on Silica                                                                              1.0        59      41                                        35   W on Silica 0          75      29                                        36   W on Silica 1.0        67      39                                        37   Fe on Silica                                                                              0          35       7                                        38   Fe on Silica                                                                              1.0        97      38                                        39   Cr on Silica                                                                              0          60      27                                        40   Cr on Silica                                                                              1.0        80      38                                        ______________________________________                                    

Data in Table V show that the use of H₂ S and high surface area contactmaterial can also give a surprising increase in cracking activity evenwhen the contact material has a catalytic metal associated therewith.Although the W sulfide catalyst of Run 36 was not as active as the Woxide catalyst of Run 35, it did provide greater selectivity topropylene.

What is claimed is:
 1. A process for cracking comprising contacting ahydrocarbon feed comprising at least one alkane having 2 to 20 carbonatoms per molecule under cracking conditions with H₂ S and a solidcontact material comprising silica having a surface area of at least 50m² /gram, wherein more H₂ S is employed than is needed for inhibitingcarbon formation under said cracking conditions.
 2. A process forcracking comprising contacting a hydrocarbon feed comprising at leastone alkane having 2 to 20 carbon atoms per molecule under crackingconditions with H₂ S and a solid contact material comprising silicahaving a surface area of at least 50 m² /gram, wherein the amount of H₂S is greater than that needed to substantially inactivate the carbonforming activity of any materials present which in the absence of the H₂S would catalyze carbon formation under said cracking conditions.
 3. Aprocess according to claim 2 wherein said cracking is carried out at atemperature in the range of about 400° C. to about 900° C.
 4. A processaccording to claim 3 wherein said cracking is conducted in the absenceof materials that are in a form that would cause any significant amountof carbon formation if said H₂ S were not employed.
 5. A processaccording to claim 4 wherein said refractory material comprises silicagel.
 6. A process according to claim 5 wherein said silica gel has asurface area in the range of about 50 m² /gram to about 350 m² /gram. 7.A process according to claim 6 wherein said refractory material consistsessentially of silica gel.
 8. A process according to claim 7 whereinsaid hydrocarbon feedstream consists essentially of n-butane.
 9. Aprocess according to claim 7 wherein said hydrocarbon feedstreamconsists essentially of n-decane.
 10. A process for cracking comprisingcontacting a feed comprising at least one alkane having 2 to 20 carbonatoms per molecule under cracking conditions with H₂ S and particles ofsilica gel having a surface area in the range of at least 50 m² /gramwherein more H₂ S is employed than is needed for inhibiting carbonformation under said cracking conditions.
 11. A process according toclaim 10 wherein said feed consists essentially of one or more alkaneseach having 2 to 12 carbon atoms per molecule.
 12. A process accordingto claim 11 wherein the major portion of the alkane in said feed isn-butane.
 13. A process according to claim 12 wherein propylene isseparated from the effluent of the cracking reaction.
 14. A processaccording to claim 11 wherein the major portion of the alkane in saidfeed is n-decane.
 15. A process according to claim 11 wherein saidalkanes of said feed are selected from the group consisting of ethane,propane, isobutane, and n-decane.
 16. A process according to claim 11wherein said silica gel has a surface area in the range of about 80 toabout 350 m² /gram.
 17. A process according to claim 16 wherein the H₂ Sis employed in an amount in the range of about 1 to about 3 mole percentbased on the total moles of alkane in said feed.
 18. A process forcracking comprising contacting a hydrocarbon feed comprising at leastone alkane having 2 to 20 carbon atoms per molecule under crackingconditions with H₂ S in a reaction zone containing a solid contactmaterial comprising silica having a surface area of at least 50 m²/gram, wherein said contact material has been contacted in said reactionzone with enough H₂ S that additional H₂ S does not provide anyadditional significant decrease in the level of carbon formation underthe cracking conditions.
 19. A process according to claim 18 whereinsaid feed consists essentially of n-butane and the product comprisespropylene.
 20. A process according to claim 19 wherein the H₂ S isemployed in an amount in the range of 0.1 to 10 mole % based on themoles of n-butane.
 21. A process according to claim 20 carried out at atemperature in the range of 500° C. to 800° C.
 22. A process accordingto claim 21 wherein said solid contact material consists essentially ofsilica.
 23. A process according to claim 22 wherein said solid contactmaterial consists essentially of silica having a surface area in therange of 80 to 350 m² /gram.
 24. A process according to claim 23 whereinpropylene is separated from the effluent of the cracking reaction.